It depends on the climates you plan on sleeping in. Some people frame the floor with 2x4 stick, spray, then lay a subfloor. Others will sandwich an inch of polyiso between the metal floor and ply sub-floor.

Many different ways to skin these cats. For retaining heat in cold weather, the ceiling and floor are the most important insulation layers. Windows can be even more important.

Put plywood on top of that and you're golden. You don't need to frame the floor first. That rigid foam can support LOTS of weight.

I screwed the plywood down, but if I were to do it again I would have used all adhesives (PL Premium). The screws transfer some cold into the bus in the winter. It isn't enough to feel, but you can see ice crystals form on their heads then when the bus warms up the ice crystals melt and make little wet spots.

I screwed the plywood down, but if I were to do it again I would have used all adhesives (PL Premium). The screws transfer some cold into the bus in the winter. It isn't enough to feel, but you can see ice crystals form on their heads then when the bus warms up the ice crystals melt and make little wet spots.